This invention relates to a rotation transmission device mounted on a part-time four-wheel drive vehicle to selectively transmit the driving torque from the engine to the non-driven wheels during a two-wheel drive mode.
FIG. 18 shows a conventional such rotation transmission device to be mounted on a four-wheel drive vehicle of a front-engine rear-drive (FR) design to selectively transmit the driving torque from the engine to the front wheels.
This rotation transmission device comprises a two-way roller clutch 72 provided between an outer ring 70 and a cam ring 71 mounted in the outer ring 70 to selectively transmit the driving torque, and a solenoid 73 for controlling the engagement of the two-way clutch 72.
The two-way clutch 72 has rollers 76 as engaging elements mounted in a wedge space defined by a cylindrical surface 74 formed on the inner periphery of the outer ring 70 and a plurality of cam surfaces 75 formed on the outer periphery of the cam ring 71. The rollers 76 are retained by a retainer 77. A switch spring 78 is mounted between the retainer 77 and the cam ring 71 to bias the retainer 77 and thus the rollers 76 to a neutral position where the outer ring 70 and the cam ring 71 are not engaged by the rollers 76.
The solenoid 73 is mounted in a recess formed in one end of the outer ring 70. Mounted between the solenoid 73 and the retainer 77 are a rotor 79 which is made of a magnetic material and is nonrotatable relative to the outer ring 70, an armature 80 which is nonrotatable but axially movable relative to the retainer 77, and a spring 81 biasing the armature 80 axially away from the rotor 79.
This rotatation transmission device is mounted on a four-wheel drive vehicle of an FR design to selectively transmit the driving torque from the engine to its front wheels. For such an application, the cam ring 71 is nonrotatably mounted on the main shaft 82 for driving the front wheels with a rolling bearing 83 mounted between the rotor 79 and the main shaft 82 to rotatably support the rotor 79.
With the rotation transmission device mounted in this way, if one or both of the rear wheels begin to rotate faster than the front wheels, the coil 73a of the solenoid 73 is energized to attract the armature 80 to the rotor 79, thereby preventing the retainer 77 from rotating relative to the outer ring 70. The retainer 77 thus rotates relative to the cam ring 71 against the force of the switch spring 78. This causes the rollers 76 to get into the wedge spaces, thereby locking the outer ring 70 and the cam ring 71 together. Torque is thus transmitted from the cam ring 71 to the outer ring 70 and then to the front wheels. This means that the vehicle has been changed over from two-wheel drive to four-wheel drive mode.
When the coil 73a of the solenoid 73 is deactivated, the spring 81 moves the armature 80 away from the rotor 79. In this state, when the outer ring 70 begins to rotate faster than the cam ring 71, the switch spring 78 disengages the rollers 76, returns them to the neutral position and keeps them there. Torque is now not transmitted from the cam ring 71 to the outer ring 70. That is, the vehicle is now being driven only by the rear wheels in a two-wheel drive mode.
In the conventional rotation transmission device shown in FIG. 18, during such a two-wheel drive mode in which the outer ring 70 stops with the rollers 76 in a neutral position, if the rotating speed of the cam ring 71 increases to a high level, since the retainer 76 rotates with the cam ring 71, the rollers 76 will move radially outwardly under centrifugal force until they physically contact the cylindrical surface 74 of the outer ring 70. Thus the cam ring 71 will be exposed to a drag force. If this drag force becomes greater than the force of the switch spring 78, the retainer 77 will rotate relative to the cam ring 71 against the force of the switch spring 78 until the rollers 76 engage in the wedge spaces, thereby locking the rings 70 and 71 together and transmitting the torque of the cam ring 71 to the outer ring 70, in spite of the fact that the solenoid 73 is not on.
One way to prevent the two-way clutch from engaging while the solenoid 73 is off is, as shown in FIG. 19, to provide a protrusion 84 on the armature 80 so as to engage in a cutout 85 formed in the cam ring 71 and thereby to prevent the armature 80 and thus the retainer 77 from rotating relative to the cam ring 71 while the solenoid is off. But when the solenoid 73 is activated and the armature 80 is attracted to the rotor 79, it is necessary to move the armature 80 axially until the protrusion 84 completely gets out of the cutout 85. Thus, it is necessary to provide an axial gap 86 between the armature 80 and the rotor 79. In order to move the armature 89 axially by the distance equal to such a large axial gap, a large-capacity solenoid is needed.
Such a large axial gap will also pose another problem that it takes a rather long time after the coil 73a of the solenoid 73 is activated until the rollers 79 actually engage, so that the rotational speed difference between the outer ring 70 and the cam ring 71 tends to grow large. Thus, a large shock may be produced when the rollers 76 engage.
A first object of this invention is to provide a rotation transmission device of the above-described type having means for preventing the rollers from displacing by a drag force and engaging into the wedge spaces defined by the cylindrical surface and the cam surfaces when the cam ring is not engaged.
Another object is to provide this type of rotation transmission device which allows easy adjustment of the axial gap between the rotor and the armature.
In a conventional such rotation transmission device, even with outer rings having a complicated shape, a predetermined permissible surface pressure is required for the engaging surface with the engaging elements. Thus outer rings are heretofore formed by forging or cutting e.g. a carburized material.
But such a manufacturing method is poor in mass-productivity and high in the product cost.
A second object of the present invention is to provide an inexpensive outer ring for a rotation transmission device having a sufficient permissible surface pressure.